Monatomic ion

Metals form cations nonmetals form anions C, P, and the metalloids do not form monatomic ions. 

The concept of oxidation number is used to simplify the electron bookkeeping in redox reactions. For a monatomic ion (e.g., Na+, S2 ), the oxidation number is, quite simply, the charge of the ion (+1, —2). In a molecule or polyatomic ion, the oxidation number of an element is a pseudo-charge obtained in a rather arbitrary way, assigning bonding electrons to the atom with the greater attraction for electrons. 

The oxidation number of an element in a monatomic ion is equal to the charge of that ion. In the ionic compound NaCl, sodium has an oxidation number of +1, chlorine an oxidation number of — 1. The oxidation numbers of aluminum and oxygen in A1203 (Al3+, O2- ions) are +3 and —2, respectively. 

With this background, we show how electron arrangements in multielectron atoms and the monatomic ions derived from them can be described in terms of—... 

Ionic radius The radius assigned to a monatomic ion, 154 main-group elements, 153t Ionic solids, 240-245 Ionization expression, 378q percent, 362... 

EXAMPLE C.i Sample exercise Identifying the likely charge of a monatomic ion... 

We recognize redox reactions by noting whether electrons have migrated from one species to another. The loss or gain of electrons is easy to identify for monatomic ions, because we can monitor the charges of the species. Thus, when Br ions are converted into bromine atoms (which go on to form Br2 molecules), we know that each Br ion must have lost an electron and hence that it has been oxidized. When 02 forms oxide ions, 02-, we know that each oxygen atom must have gained two electrons and therefore that it has been reduced. The difficulty arises when the transfer of electrons is accompanied by the transfer of atoms. For example, is chlorine gas, Cl2, oxidized or reduced when it is converted into hypochlorite ions, CIO" ... 

The oxidation number of an element in a monatomic ion is the same as its charge. For example, the oxidation number of magnesium is +2 when it is present as Mg2+ ions, and the oxidation number of chlorine is — 1 when it is present as Cl" ions. The oxidation number of the elemental form of an element is 0 so magnesium metal has oxidation number 0 and chlorine in the form of Cl2 molecules also has oxidation number 0. When magnesium combines with chlorine, the oxidation numbers change as follows ... 

Formulas of compounds consisting of the monatomic ions of main-group elements can be predicted by assuming that cations have lost all their valence electrons and anions have gained electrons in their valence shells until each ion has an octet of electrons, ora duplet in the case of FI, Li, and Be. 

FIGURE 2.8 A micrograph ot bone, which owes its rigidity to calcium phosphate. The overlay shows part of the crystal structure of calcium phosphate. Phosphate ions are polyatomic ions however, as shown in the inset, they are nearly spherical and fit into crystal structures in much the same way as monatomic ions of charge —3. 

On the basis of the expected charges on the monatomic ions, give the chemical formula of each of the following compounds (a) magnesium arsenide (b) indium(III) sulfide ... 

Hund37 in 1926 and of oxygen and nitric oxide by Van Vleck38 in 1928 were triumphs of the theory of spectra. The magnetic moment of an atom or monatomic ion with Russell-Saunders coupling of the quantum vectors is... 

The oxidation number of a monatomic ion in an essentially ionic substance is equal to its electrical charge. 

The forces between ions have been discussed by Lennard-Jones and his collaborators, who have given tables showing the repulsive forces as a function of the repulsion exponent n [Lennard-Jones and Dent, Proc. Roy. Soc., 112A, 230 (1926)]. In conjunction with Wasastjema s radii, these tables have been used in the theoretical treatment of crystals such as caldte, CaCOj, which, however, we consider not to be composed of monatomic ions. Thus, they assume C+1 and O" to be present in caldte [Lennard-Jones and Dent, Proc. Roy. Soc., 113A, 673, 690 (1927)], although the carbonate ion is generally believed by chemists to contain shared-electron bonds. 

Many complex ions, such as NH4+, N(CH3)4+, PtCle", Cr(H20)3+++, etc., are roughly spherical in shape, so that they may be treated as a first approximation as spherical. Crystal radii can then be derived for them from measured inter-atomic distances although, in general, on account of the lack of complete spherical symmetry radii obtained for a given ion from crystals with different structures may show some variation. Moreover, our treatment of the relative stabilities of different structures may also be applied to complex ion crystals thus the compounds K2SnCle, Ni(NH3)3Cl2 and [N(CH3)4]2PtCl3, for example, have the fluorite structure, with the monatomic ions replaced by complex ions and, as shown in Table XVII, their radius ratios fulfil the fluorite requirement. Doubtless in many cases, however, the crystal structure is determined by the shapes of the complex ions. 

The molal diamagnetic susceptibilities of rare gas atoms and a number of monatomic ions obtained by the use of equation (34) are given in Table IV. The values for the hydrogen-like atoms and ions are accurate, since here the screening constant is zero. It was found necessary to take into consideration in all cases except the neon (and helium) structure not only the outermost electron shell but also the next inner shell, whose contribution is for argon 5 per cent., for krypton 12 per cent., and for xenon 20 per cent, of the total. 

A number of general features in Table 1-3 is apparent. Complexes may be cationic, neutral or anionic. Ligands may be simple monatomic ions, or larger molecules or ions. Many ligands are found as related neutral and anionic species (for example, water, hydroxide and oxide). Complexes may contain all of the same type of ligand, in which case they are termed homoleptic, or they may contain a variety of ligand types, whereby they are described as heteroleptic. Some ligands such as nitrite or thiocyanate can coordinate to a metal ion in more than one way. This is described as ambidentate behaviour. In such cases, we commonly indicate... 

The oxidation number of every monatomic ion is equal to its charge. 

Ans. Rule 2 Uncombincd elements have zero charges, and so the oxidation numbers must add up to 0. Since all the atoms are the same, all the oxidation numbers must be the same—0. Rule 3 Monatomic ions have the oxidation numbers of all the atoms add up to the charge on the ion. Since there is only one atom (monatomic), the oxidation number of that atom must add up to the charge on the ion that is. it is equal to the charge on the ion. 

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